This invention relates to a novel article of manufacture carrying an electrically-resistive glaze whose properties are stable in high-electric fields, and to methods for preparing said article particularly for use in an electrical device.
There are many applications in which a body carrying an electrically-resistive glaze operates either continuously or intermittently in a high-electric field; that is, a field of 10 kilovolts per centimeter or higher. In one application, for example, a resistive glaze on a ceramic substrate is used in an electron gun for a cathoderay tube to provide a graded or distributed electric field (or electronic lens) for acting on an electron beam. In some forms of this gun, a resistive glaze is coated upon an insulating support such as a ceramic body, and the glaze distributes the voltage along the beam path either directly or through spaced conductors. These latter structures are sometimes referred to as "resistive lenses."
In this and similar applications, the resistive glaze must have a particular combination of properties which are not available with known prior glazes. Besides the usual requirements of low cost and ease of fabrication, the resistive glaze must have a sheet resistance in the range of about 0.5 .times. 10.sup.8 to 500 .times. 10.sup.8 ohms per square, a resistance variation with temperature characterized by an activation energy of less than 0.1 eV, and volume and sheet resistivities which are substantially constant in electric fields up to about 30 kilovolts per centimeter for substantial periods of time at temperatures up to 200.degree. C. In this specification, the values of sheet resistance are for layers which are about 0.01 centimeter thick. To convert these values of sheet resistance to resistivities in ohm-centimeters, the values of sheet resistance are divided by 100.
High-tension insulators comprising ceramic bodies carrying a resistive glaze are described in British Pat. No. 982,600 to D. B. Binns, in U.S. Pat. No. 3,795,499 to Y. Ogawa et al. and in D. B. Binns, Transactions of the British Ceramic Society 73 7-17 (1974). Generally, the resistive glazes described in these publications consist essentially of a nonconducting glass matrix containing a conducting network of metal-oxide particles, which particles have, prior to incorporation in the glaze, been suitably doped with impurity ions to enhance the conductivity of the particles. In one family of glazes, tin-oxide particles are doped with antimony oxide as by calcining, and then the doped tin-oxide particles are mixed with an ordinary glass, such as a soda-lime glass or a lead glass, and the mixture is coated and melted to produce the glaze. The sheet resistance of the glazes can be varied within limits by varying the weight ratio of doped tin-oxide particles to glass and by varying the mol ratio of antimony oxide to tin oxide in the doped tin-oxide particles. At low electric fields (less than 1 kilovolt/cm), sheet resistances are reported to be in the range of 10.sup.7 to 10.sup.10 ohms per square. However, our measurements indicate that, at high-electric fields (10 kilovolts per centimeter and higher) and elevated temperatures, these glazes deteriorate rapidly. For example, after about one hour at about 200.degree. C with a field of 20 kilovolts per centimeter applied, one glaze showed discoloration, pitting and an increase in resistance by a factor of three.